The present invention relates generally to a system for reproducing video signals from a magnetic tape for obtaining a slow-motion reproduction picture, and more particularly to a system for carrying out slow-motion reproduction so as to obtain a stable reproduced slow-motion picture without any accompanying noise therein and oscillation or wobbling thereof.
In a helical scanning type of video signal recording and reproducing apparatus, the video signals are successively recorded by one or more rotary heads on tracks oblique to the longitudinal line of the tape, such that, for example, one field is recorded per track. For slow motion reproduction of these recorded video signals, the tape is driven at a speed which is slower than a speed in the recording mode (or normal reproducing mode). Accordingly, in reproduction, the rotary heads repeatedly trace the respective tracks a plurality of times, whereby the reproduced picture has a slower motion than it would have if reproduced in a normal reproducing mode. It is in this way that slow-motion reproduction is achieved.
In the slow-motion reproducing mode, the rotary heads rotate at the same speed as in normal reproduction mode, while the tape is driven at a speed slower than that in a normal reproducing mode. As a result the inclination angle of the track traced by the rotary head on the tape in slow-motion reproduction mode differs from the inclination angle of the track in a recording mode (or normal reproduction mode) and this difference in inclination gives rise to tracking deviation.
In reproduction, when the rotary heads deviate from the signal tracks on the tape, a noise bar is generated in the reproduced picture. In the conventional slow-motion reproduction, the tape is continuously driven at a speed slower than the normal traveling speed at the time of a normal reproducing mode. The position at which the rotary heads deviate from the tracks undergoes successive shifts. For this reason, the position of the noise bar continuously shifts in the reproduced picture, (from top to bottom, for instance) and the noise therefore impairs the quality of the entire reproduced picture.
A DC motor is generally used for rotating the capstan so as to make it possible to drive the tape at different speeds. When the DC motor is to be rotated slowly for slow speed tape travel, the voltage applied to the DC motor is lowered to a potential which is less than the potential used at the time of normal rotation. However, when the voltage applied to the motor is lowered, the rotational torque of the motor decreases, and as a result the tape cannot be driven stably.
Moreover, when the DC motor is to be rotated at a slow speed, it is necessary to overcome starting friction by first applying to the motor a higher voltage, relative to the voltage required to obtain the desired low speed of rotation. Then, after the motor has started the voltage is reduced to the value required for slow speed rotation. However, the raising and lowering of the voltage applied to the DC motor to change the speed of rotation gives rise to hysteresis thus introducing a complex factor into rotation speed control and adjustment.
Accordingly, in order to overcome the above described difficulties, there have been previously proposed "TAPE DRIVING SYSTEM IN A RECORDING AND/OR REPRODUCING APPARATUS" in U.S. patent application Ser. No. 908,917, filed May 24, 1978, now U.S. Pat. No. 4,190,869, issued Feb. 26, 1980, and "SYSTEM FOR REPRODUCING A VIDEO SIGNAL IN A SLOW MOTION OR STILL PICTURE REPRODUCTION" in U.S. patent application Ser. No. 9077, filed Feb. 2, 1979, allowed on June 3, 1980, now U.S. Pat. No. 4,246,616 issued Jan. 20, 1981.
In these proposed systems, the tape makes intermittent shifts. Between shifts, when the tape is stopped, the same track is traced by the rotary heads a plurality of times and in this way slow-motion reproduction is carried out. Accordingly, it becomes possible to obtain the slow-motion reproduction picture with little accompanying noise. When the tape is being shifted, noise is generated for a small period of time. However, this noise can be pushed into an inconspicuous part of the reproduced picture so that the viewer can enjoy good quality slow-motion reproduction picture. Moreover, when averaging over a period of time which is long relative to frequency of applying a high voltage to start up the DC motor, the motor actually performing intermittent rotation can be considered as performing ordinary slow speed rotation for driving the tape. Accordingly, there is no decrease in the rotational torque of the motor, as there is in the prior system.
However, during the above mentioned slow-motion reproduction, the relative tracing speeds of the rotary video heads vary relative to the tape respectively at the time when the tape is stopped and at the time it is traveling are different. Although this relative tracing speed differs with the direction of tracing of the rotary video heads relative to the tape traveling direction, the relative tracing speed at the time of tape travel becomes lower by approximately 0.5 percent, as one example, in comparison with that at the time the tape is stopped. This variation in the relative tracing speed gives rise to variations in the periods of the horizontal synchronizing signal and the vertical synchronizing signal of the reproduced video signal. As a consequence, there has heretofore been the problem of intermittent oscillation of the reproduced image in the horizontal and vertical directions.